1. Field of the Invention
This invention relates to a clamping device, especially, but not exclusively, for clamping work tables, fixtures or the like to a base in e.g. machine tools, comprising a holding means having an enlarged head, clamping means which in their clamping position engage the enlarged head of the holding means, and a clamping mechanism which in its normal condition applies its full clamping force to the clamping means, but which unloads the clamping means when an disengagment force is applied thereto. Moreover the invention relates to a method and an arrangement for combined cleaning and accuracy control by a clamping device of the kind indicated above.
2. Description of the Prior Art
All of the clamping devices presently available on the market suffer from at least some of the following disadvantages: The devices comprise a great number of parts having such exposed positions that they become sensitive to fouling and wear, in connection with machining by detachment of cuttings there is also a great danger that cuttings may assemble on the different parts and prevent a correct clamping; the devices require much space and this means that the work part (e.g. a fixture) that is to be clamped must be oversized in order to accomodate the clamping devices, and moreover it is difficult to install these clamping devices in e.g. an existing machine tool; they comprise at least some projecting part which makes it impossible to transport the work piece to be clamped on e.g. roll conveyors, without damaging said projecting part, the clamping devices are in themselves, i.e. irrespective of the modifications that must be carried out on the work part to be clamped or on the existing machine tool, very expensive to manufacture. Finally these known clamping devices often require the use of a separate hydraulic unit for applying the necessary clamping force.
An example of such a known clamping device is that commonly used for clamping pallets or fixtures by machining centres. This known device comprises a clamping plate intended to be inserted into a substantially T-shaped recess at the bottom of the pallet. The clamping plate is firmly connected to a cylinder housing in which a piston is provided which seals against the inner wall of the cylinder housing and which is stationary in relation to the machine table of the machining centre, but which is slidable in relation to the cylinder housing, whereby the cylinder housing and the clamping plate connected thereto are movable up and down in the clamping direction when pressurized fluid is introduced at the upper side and lower side respectively of the piston. When clamping a pallet the clamping plate engaging the T-shaped recess assists in bringing the pallet to a position engaging a number of stops provided on the machine table. This known device suffers from almost all of the above mentioned disadvantages. Thus the T-shaped recess in the pallet is extremely exposed to fouling and cuttings which are difficult to remove efficiently gather in this recess as well as on the clamping plate. It frequently happens that such accumulations of cuttings bring the machining centre to a standstill whereupon the pallet and the clamping device have to be manually cleaned from these cuttings.
Another known clamping device of the kind in question is almost impossible to install in existing machine tools due to its bulky configuration, and even if such an installation could be possible it would require a hydraulic unit provided in connection thereto since the clamping device demands relatively large amounts of hydraulic fluid for the maneuvering thereof. Finally this known clamping device requires that the pallet or fixture is provided with the above mentioned T-shaped groove at its bottom and as discussed above this means that the pallet becomes impossible to transport on e.g. a roll conveyor. This special design of the pallet brings about the disadvantage that this as well as the clamping device in itself becomes much more expensive.
However, through Swedish printed and published specification No. 7804353-6 there is previously known a clamping device of the kind mentioned initially, which eliminates one or some of the above discussed disadvantages. This last mentioned, known clamping device is however, designated for a very specific and narrowly delimited range of application since it is adapted for clamping a two-part tool in a press or injection machine or the like. More specifically this means that the clamping device consists of an upper and a lower clamping unit, i.e. one unit for the upper tool and one unit for the lower tool arranged at a distance from the upper tool. It is however possible to use these upper and lower clamping units one by one for other clamping purposes, but even in such a case these suffer from very serious shortcomings and limitations regarding the use thereof.
Thus, said lower clamping unit comprises a holding means consisting of two elongate guide bars, while the clamping means consists of a pin having a head and being connected to the clamping mechanism so that it, when it has been inserted sideways into the groove formed by the guide bars, engages the guide bars with its head when the clamping mechanism is maneuvred. Like the above described clamping device for pallets this lower clamping unit suffers from the disadvantage that the guide bars (which to a great extent correspond to the T-shaped groove in the pallet) render a transportation of the tool on a roll conveyor impossible. Dirt and cuttings may also very easily get stuck in the guide bars so that they prevent clamping. As mentioned above it is also very difficult to perform an effective cleaning of such an elongate guide groove. Moreover the complete clamping unit, in the clamping position also the guide bars, is provided in or below the machine table and this means that it would be very complicated and unacceptably expensive to modify an existing machine tool so that this clamping unit could be employed.
The upper clamping unit likewise comprises a holding means consisting of two elongate guide bars attached to the rear side of the tool and involving the above discussed disadvantages. However, in this case the clamping means consists of a pin having two opposite projections at its outer end. After the tool has been brought down against the work table, but prior to the application of the clamping force, the clamping means is rotated so that its projections engage the recess or groove of the guide bars, and this is a long known principle for the clamping of work pieces on a work table by means T-shaped grooves. The disadvantage of this solution is that very bulky operating devices are required for rotation the clamping means. Moreover the disclosed solution, where the clamping mechanism consists of a piston against which cup springs act, is most inappropriate in this case since the piston is rotated together with the clamping means and consequently is subject to a very hard wear of the surface facing the cup springs. Otherwise this upper clamping unit to an even greater extent suffers from the same disadvantages as the above described lower clamping unit.